Oceanic flights use Air Traffic Control (ATC) datalink applications called Future Air Navigation System (FANS) 1/A. Currently, such ATC datalink applications are required in some oceanic regions like the north Atlantic. The use of these ATC datalink applications begins while the aircraft is still within range of land based datalink Very High Frequency (VHF) ground stations, which use the Aircraft Communications Addressing and Reporting System (ACARS), such as Plain Old ACARS (POA), or VHF DataLink (VDL) mode 2 ACARS over Aviation VHF Link Control (AVLC) (AOA).
As the aircraft flies into remote areas without VHF coverage (like northern Canada) or out over the ocean, the datalink communications transition to datalink satellite communications (SATCOM). The datalink communications transition logic is reactive in current avionics systems. When an avionics system sends a downlink message via VHF and does not receive a response, even after performing all of the retries, then the downlink message is transmitted via SATCOM. This retry process can take longer than 90 seconds. The required communication performance (RCP) for Automatic Dependent Surveillance-Contract (ADS-C), one of the FANS 1/A applications, is that 95% of the downlink reports must be received within 90 seconds.
The Federal Aviation Administration (FAA) monitors the RCP of the aircraft in FAA oceanic airspace. Problem reports are written against aircraft that consistently fail to meet the RCP of 95% within 90 seconds, and the aircraft will be denied use of preferred tracks, which will increase the flight time and cost (e.g., increased crew pay, increased fuel cost). A typical flight across the north Atlantic generates 10-20 ADS-C reports. If one or more of the reports is delayed, then the aircraft will not meet RCP.
While there are various causes for report delays, such as satellite switching, multiple downlink reports, etc., the VHF to SATCOM transition is the biggest cause of delays. Some geographic regions have offered to help with the transition, but it is difficult to accurately define the boundary for the transition (e.g., coastlines are very irregular). The VHF coverage along coastlines depends on where the VHF ground stations are located. In addition, coverage changes as new VHF ground stations are deployed and old VHF ground stations are decommissioned. Any maps of such ground stations would need to be tailored for each service provider. Another problem is that switching to SATCOM early will increase the communication cost, because SATCOM is significantly more expensive than VHF.